On the rarity of these formations, the AMS stated: “Despite their aesthetic appearance, mammatus have been the subject of few quantitative research studies,” and that “observations have been obtained largely through serendipitous opportunities.”

Weather.com states that mammatus clouds are formed when ice crystals fall from the cumulonimbus cloud’s anvil. “The ice crystals sublimate, or change from ice to water vapor as they fall, causing the surrounding air to thermodynamically cool. The cooled air becomes negatively buoyant and begins to sink, producing the punched-out look indicative of the mammatus cloud.”

To be sure, the precise dynamics are complex, with an end result that can be both menacing and surreal.

Stated Last, the NOAA meteorologist, via email: “They are sometimes ominous in appearance, but are harmless and do not mean that severe weather is necessarily going to occur. In fact, they are usually seen after the worst of a storm has passed.”